Google uses both schema.org markup and og:image meta tag for thumbnails in Google Search and Discover

Google clarified this within its Google image best practices and Google Discover documentation.

Google Clarifies How It Picks Thumbnails For Search, Discover

Google updated its Image SEO and Discover documentation to explain how schema markup and the og:image meta tag influence ...
searchenginewatch

The State of Schema.org: What are the biggest challenges surrounding Schema markup?

Using Schema.org markup, a form of structured data which helps search engines to interpret your webpages, is widely agreed to be beneficial from an SEO standpoint. While it may not correlate directly ...